In a copending application of Norman L. Stauffer, Ser. No. 85,821, filed Oct. 17, 1979, now U.S. Pat. No. 4,309,603 a low cost auto focus system is disclosed in which as few as two radiation responsive detector pairs are utilized thus avoiding the necessity for a large and costly detector array that had been common in the prior art at that time.
In a copending application of Dennis J. Wilwerding, Ser. No. 99,235, filed Nov. 30, 1979, now U.S. Pat. No. 4,309,603 an improvement on this basic low cost auto focus system was disclosed in which a bias signal was introduced to prevent improper zero crossover situations that could occur during very low contrast conditions.
In copending application of J. Frazier, Ser. No. 212,930, filed Dec. 4, 1980, an improved low cost auto focus system is disclosed which examined the absolute values of log a.sub.1 /b.sub.1 and log a.sub.2 /b.sub.2, where a.sub.1, b.sub.1, a.sub.2 and b.sub.2 are the output signals from the detectors and, upon determining the largest absolute value, causes the auto focus circuitry to respond to the largest log signal. As part of the circuitry discussed in that application, a default circuit is disclosed which operates to stop the focus lens at the infinity or hyperfocal extension point when the scene being viewed has too little contrast to produce a satisfactory focus signal.
In a copending application of John Wittenberger, Ser. No. 212,918, filed on Dec. 4, 1980, a further improved low cost auto focus system is disclosed which involves a comparison of the absolute magnitude of log a.sub.1 /b.sub.1 +log a.sub.2 /b.sub.2 with the absolute magnitude of log a.sub.2 /b.sub.2 -log a.sub.1 /b.sub.1 where a.sub.1, b.sub.1, a.sub.2 and b.sub.2 are the output signals from the two pairs of radiation detectors. As part of the circuitry discussed in that application, a default logic circuit similar to that disclosed in the above-mentioned copending application Ser. No. 212,930 is disclosed which also operates to prevent the system from operating normally in the event that there is insufficient contrast.